We have shown that 1.2 grams of aspirin daily, started preoperatively, provides a safe and effective (albeit incomplete) prophylaxis against venous thromboembolic disease in men over the age of 40, who have not had knwon venous thromboembolic disease, when they undergo total hip replacement. Women are not protected. The major objectives of our continuing study are two-fold: a) to further evaluate aspirin prophylaxis against venous thromboembolic disease; b) to study the true incidence of pulmonary emboli usingtheCO2 pulmonary scan. We propose to continue our study of 1.2 grams of aspirin daily in both men and women, and in both sexes with and without known prior thromboembolic disease. We are comparing that against the efficacy and safety of 3.6 grams daily. We are also doing routine C15O2 pulmonary scanning preoperatively and every four days postoperatively to gain important insights into two heretofore unresolved vital aspects of the problem of venous pulmonary emboli, namely, a) the prevalence of clinically silent emboli following THR and b) by correlation with the routine bilateral venography done in all patients in this study, the prevalence, location, magnitude and time of onset of pulmonary emboli (both silent and clinically apparent) in relation to the location of venous thrombi in the lower extremeities. To date, 3.6 grams of aspirin does not appear to be an advantage over 1.2. This is in distinct contrast to the recent publication by McKenna et al (1), in which 975 milligrams of aspirin was not effective at all but 3900 milligrams of aspirin gave significant and striking protection against deep vein thrombosis development in a group of patients undergoing total clinical aspects, further pursuit of the use of aspirin as a prophylactic agent is essential. In addition, the new insights which will be provided by the routine CO2 scans will be extremely helpful in understanding venous pulmonary emboli.